The present invention relates generally to wireless communication networks, and more particularly, to systems and methods for adapting the capacity or maximum throughput of a wireless network based on a load on the network.
A wireless network generally includes a group of base stations interconnected by transmission circuits, switching elements, and mobile terminals that provide service to nomadic users. As an example, FIG. 1 illustrates a simplified wireless network 100 that includes base stations 105-109, mobile terminals 110-116, and a switching element 130. The nominal coverage area of each base station 105-109 is often referred to as a cell. For example, cell 117 represents the nominal coverage area of base station 105, cell 118 represents the nominal coverage area of base station 106, and so on. Communication between base stations 105-109 and mobile terminals 110-116 takes place over the air interface, which is a radio link with a specified set of parameters.
Two important characteristics of a wireless network are capacity and coverage. Capacity may be defined as the maximum throughput (in bits per second) per base station. Coverage may be defined as the fraction of the nominal service area or cell over which service can be obtained.
Technology has made it increasingly important for base stations and mobile terminals to be able to communicate at relatively high rates. For example, the rates supported by third-generation (3G) cellular and personal communications systems greatly improve on the data transfer capabilities of second-generation systems. These rates, however, will not support the high speed network connectivity to which users are becoming accustomed as landline network technology evolves. Moreover, even the use of these rates will tax the capacity of a 3G cell. In the long term, there is a need for another solution to provide high speed wireless networking to support data-rate-intensive communications, such as multimedia applications and large file transfers.
To meet the demand for higher data rates, wireless networks need to maximize their capacity or throughput. Traditional public wireless networks are typically designed to provide uniform coverage over the entire area of the cell with a fixed upper limit on total data throughput. By relaxing the uniform coverage constraint, it is possible to increase the capacity of the network.
Many modern digital wireless air interfaces have the capability to adjust the transmit power and/or bit rate on a given link so that the transmitted energy per bit, denoted Ebt (joules per bit) is the minimum necessary to support the link. The greater the distance between the base and the mobile, the higher the required Ebt. If the total maximum base station transmit power is P (watts) and the average required energy per transmitted bit, for all mobiles served by the base station, is Eavg (joules per bit), the total throughput for the base station is R=P/Eavg (bits per second). Since P is typically fixed, the total throughput is maximized by minimizing Eavg. Serving mobiles far away from the base station greatly increases Eavg and therefore reduces the throughput R. By restricting service to mobiles near the base, the throughput can be increased at the expense of coverage. An inherent tradeoff therefore exists between capacity and coverage.
From the perspective of throughput, the Ebt required to serve a given mobile represents the system resource cost of serving that mobile. If Pm is the transmit power allocated to that mobile and Rm is the rate of the transmission to that mobile, in bits per second, then Ebt=Pm/Rm. The required Ebt therefore can be realized by allocating different power levels to different mobiles (dividing the total power among all served mobiles), or by serving different mobiles sequentially in time using the same total power but different bit rates, or by a combination of power control and rate control (adaptive rate modulation). Referring again to FIG. 1, mobile terminal 116, which is near base station 109, may have a low Ebt and therefore may communicate with the base station 109 at a relatively low cost, whereas mobile terminal 114, which is away from base station 109, may communicate with the base station 109 at a high cost. One known method for increasing the average throughput or total capacity of the base station is to selectively exclude or deny service to high-cost mobiles and serve only those mobiles with Ebt below some threshold value. However, this is only productive in terms of increasing throughput if the total demand for service (offered load) exceeds the level that can be supported by the base station.
A known method relaxes the uniform coverage constraint to increase capacity. By limiting coverage to a small area around each base station, the mobile terminals being served may have a lower required Ebt, thus permitting a greater throughput. This is the basis of the Infostations concept, described in R. H. Frenkiel, et al., “The Infostations Challenge: Balancing Cost and Ubiquity in Delivering Wireless Data,” IEEE Pers. Commun. Mag., pp. 66-71, April 2000. Frenkiel proposes combining the downlink-only coverage zones surrounding base stations. The modulation rates in this approach would adapt within limits to maximize the data transmission rate, based on the mobile's required Ebt. One limitation of this approach is potential wasted air space or unused system resources when few mobiles are in the coverage zones.
While the above-described method and the methods of existing wireless networks focus on maximizing capacity subject to one or more constraints, the capacity itself is static, in that these methods are not responsive to load presented on the network. Instead, these methods share the available capacity among active users that are distributed over a fixed coverage area in some fashion. This static approach does not allow the inherent capacity/coverage tradeoff to be exploited in an adaptive manner as the load or demand for service changes.